This invention relates generally to measuring light intensity uniformity in apparatus for exposing a light-sensitive object to a succession of light images in closely controlled positions. This type of apparatus finds particular use in the manufacture of integrated circuits. In one method of manufacturing such integrated circuits, an oxidized wafer of silicon is coated with a photo-resist material. The wafer may be of such dimensions that, after processing, it can be cut along rows and columns into a large number of chips. When such coated wafers are exposed to light, the photo-resist material polymerizes into a tenacious coating, whereas the unexposed portions of the layer are readily removable, as for example, by means of a solvent or developer. By means of a "mask" or "master", which is opaque except for a pattern of transparent lines and areas therein through which light can pass, the wafer with photo-resist coating is exposed to light in a pattern conforming to elements of a circuit to produce a photographic reverse thereof. The mask typically bears a multiplicity of the same pattern arranged in rows and columns. An image of the mask may be projected by image forming means onto the surface of the wafer as in projection printing. An apparatus for such projection printing is shown and described in U.S. Pat. No. 4,068,947. In this patent, an optical apparatus is shown and described wherein a wafer may be exposed to the image of a mask through use of a pivotal carriage. In such an apparatus, it is important to regulate the light intensity uniformity because the fidelity of geometries produced in the photoresist depend on it. The slit through which the light passes must be properly tapered in accordance with the relative distance from the pivot axis of the carriage and different positions along the slit. As a result of this taper a light detector scanned along the slit would ideally see a linear variation of intensity as it traversed from one end of the slit to the other. For checking the variation from this ideal it is advantageous to eliminate the linear variation from the intensity trace and provide a uniformity reading by an analogue or digital panel meter.